Hydrocarbon-producing wells are often stimulated by hydraulic fracturing treatments. In hydraulic fracturing treatments, a viscous fracturing fluid, which also functions as a carrier fluid, is pumped into a producing zone to be fractured at a rate and pressure such that one or more fractures are formed in the zone. Particulate solids for propping open the fractures, commonly referred to in the art as “proppant,” are generally suspended in at least a portion of the fracturing fluid so that the particulate solids are deposited in the fractures when the fracturing fluid reverts to a thin fluid to be returned to the surface. The proppant deposited in the fractures functions to prevent the fractures from fully closing and maintains conductive channels through which produced hydrocarbons can flow.
After the fracturing fluid, which is the carrier fluid for the proppant, deposits the proppant in the fracture, the fracture closes on the proppant. Such partially closed fractures apply pressure on proppant particles. For this purpose, the interstitial space between particles should be sufficiently large, yet possess the mechanical strength to withstand closure stresses to hold fractures open after the fracturing pressure is withdrawn. Thus, for instance, large mesh proppants exhibit greater permeability than small mesh proppants at low closure stresses, but they will mechanically fail and thereby produce very fine particulates (“fines”) at high closure pressures.
Modifications of proppant particles could be used advantageously to improve their performance in hydraulic fracturing systems. First, if the proppant particles were less dense, a less viscous fracturing fluid could be used, which would still convey the particles to the target area but which would be easier to pump into the formation. Second, proppants should remain where they are placed throughout the lifetime of the well after they have been injected into a fracture line. If changes within the reservoir during well production force the proppants out of position, production equipment can be damaged, and the conductivity of the reservoir formation can be decreased as the reservoir pores are plugged by the displaced proppants. Third, the proppants in the system should be resistant to closure stress once they are placed in the fracture. Closure stresses can range from 1700 psi in certain shale gas wells, up to and exceeding 15,000 psi for deep, high temperature wells. Care must be taken that the proppants do not fail under this stress, lest they be crushed into fine particles that can migrate to undesirable locations within the well, thereby affecting production.
Sand is a common proppant, though untreated sand is prone to significant fines generation. Alternative proppant materials include ceramics and sintered bauxite.